In the field of high speed data communications, modems employed for transmitting information often are of the type which perform multi-amplitude signalling, wherein plural amplitude levels are used to designate respective information states. Where telephone lines and associated telephone equipment form part of the transmission medium, amplitude fading and distortion constitute significant impairment to the accurate reconstruction of transmitted signals, and the higher the transmission rate the more complex the problem becomes. A common piece of receiver equipment in amplitude modulation signalling systems is gain correction or gain control circuitry which monitors variations in the amplitudes of received signals and adjusts the received signal amplitude in accordance with some preestablished gain control function. Because of the wide variety of modulation schemes which may be adopted, the gain adjustment circuitry is normally tailored for a particular data modulation technique.
For example, where data is transmitted in binary or digital format, the receiver circuitry will commonly incorporate threshold detection circuitry with reference level adjustments to provide corrective action for amplitude tracking. One such system is described in the U.S. patent to Sant'Agostino U.S. Pat. No. 4,034,340 which relates to a system which monitors multi-amplitude signals, such as binary or digital messages, and compares the received signal amplitudes at prescribed time intervals with established threshold levels. There is a maximum level and a minimum level relative to which pseudo-error values may be defined. The maximum and minimum levels themselves define a medium threshold relative to which a signal sample is determined to be either a "1" or a "0." A comparator network is employed to determine whether or not one of threshold values should be moved to maintain a substantially constant pseudo-error rate.
The U.S. patent to Gibson U.S. Pat. No. 3,736,511 describes a further type of adjustable threshold decision circuit, wherein signal samples are compared against a set of adjustable reference levels and a decision is made as to the value of each signal sample as well as the sign of the sample. Adjustment of the threshold levels is made in accordance with a set of algorithms which effectively take the difference between a set of samples and store the same.
Another threshold adjustment system is described in the U.S. patent to Sullivan U.S. Pat. No. 3,665,326 which relates to the control of the threshold level in a digital data sampling system to maintain a prescribed percentage of "1" bits.
In addition to particularly tailored threshold adjustment amplitude tracking circuitry, there are known amplitude detection circuits which vary the gain in accordance with amplitude deviations from established zones or regions. For example, the U.S. patent to Ragsdale U.S. Pat. No. 3,619,503 describes a technique of overcoming distortion in a multi-amplitude, multi-phase telephone transmission line-coupled modem, wherein upper and lower ranges for establishing amplitude levels are employed for the control of the gain of the system depending upon the amplitude of the signals relative to those ranges and logic levels separating the ranges from each other. If the amplitude of the signal is fairly close to one of the logic levels in the intermediate ranges, the gain is adjusted in the appropriate direction to bring the amplitude of the signal to the prescribed logic level. If the amplitude falls in an outer level, namely the amplitude level is extremely large, there is a rapid decrease in the gain level.
Another type of gain control system is described in the U.S. patent to Conway U.S. Pat. No. 3,851,266 which relates to a system for adjusting the gain and DC level of a signal measuring circuit in accordance with whether or not a signal exceeds or is less than maximum and minimum threshold values. If a signal sample is positive and greater than some upper threshold level, the gain is reduced; however, if the positive sample is less than this upper threshold, the gain is increased. If, on the other hand, the signal sample is negative and greater, in absolute value, than the minimum threshold representing the lower level, a positive bias is added to increase the DC level. A technique similar to this is discussed in the U.S. patent to Tracey et al U.S. Pat. No. 3,864,529 which shows another type of multi-amplitude signal processing system wherein the gain of the system is adjusted depending upon where the amplitude of a signal sample falls. An amplitude scale is broken-up into zones and either increase or decrease gain signals are generated depending upon whether or not the signal amplitudes exceed or are less than zone threshold levels.
In each of the above systems, the gain adjustment technique described is particularly related to the modulation and transmission scheme employed and may not have wide application or be suited to other types of transmission systems wherein the type of modulation employed or the data rate are considerably different. Moreover, in prior art amplitude tracking schemes which use adjustment error signals from decided upon amplitudes as a basis for adjusting gain, there exists the problem of hang-up modes rendering such techniques substantially useless.